1. Field of this Invention
This invention relates to a multiple waveband optical element, specifically but not exclusively to a window for use in the infra-red and millimetric wavebands.
2. DISCUSSION OF PRIOR ART
Most infra-red window materials have low loss in the microwave (8-16 GHz) and millimetric (35 and 95 GHz) bands and can be used as a window provided the reflection can be kept to a minimum. The reflection is a minimum if the window is an integral number of half wavelengths thick. For frequencies up to 35 GHz the required tolerances on thickness are easily attainable. At 95 GHz however, the stricter tolerances required are very difficult to achieve.
In order to reduce the reflection from optical components it is known to provide them with anti-reflection coatings, which coatings may comprise several layers of chosen thicknesses and refractive indices. European patent application No. 2149138 discloses such a multilayer anti-reflection coating intended to provide an anti-reflection effect within two separated waveband regions. A problem with such coatings, i.e., thin films of material deposited on the basic optical component by evaporation or a like technique, is that it is difficult or even impossible to achieve layer thicknesses of the order required to provide a truly worthwhile anti-reflection effect in the millimetre waveband.